nth and Penfidd — Lansfordite^ Nesguehonite, <t<-. 121 



treme difference in path of about 250,000 waves is about one 



part in half a million. 



It is believed that in some experiments recently published* 

 this degree of accuracy has been attained with a difference of 

 path only one-tenth as large, and exceeded with twice this 

 number. But there is no reason to think that (in some ca 

 at least) the limit o{' visible interferences will he reached even 

 with two or three hundred thousand waves. This means ulti- 

 mately an error of something like <>/<< part in twenty mi/I 'ion* 

 in the estimate of a single wave-length. 



Thus it appears that it is possible to construct a single instru- 

 ment of a few pieces of plane ^'lass which can combine the 

 functions of a microscope, a telescope and a spectroscope; and 

 thatyor purposes of measurement may be made to far surpass 

 these instruments in accuracy. 



Art. XIX. — On Lansfordite, Nesquehonite, a new Mineral, 

 domorphs of JVesquehonite after Lansfordite ; by 

 F. A. Genth and S.'L. Penfield. With Plate IV. 



Under the name lansfordite one of usf described a new 

 mineral which had been discovered in October, 1887, by 

 Messrs. D. M. Stackhouse and F. J. Keeley in one of the an- 

 thracite mines in the neighborhood of Lansford, near Tama- 

 qua, Schuylkill Co., Pa. The specimens collected at that time, 

 which were entirely uniform in appearance, occurred as incrust- 

 ations and in the form of stalactites showing some crystalline 

 faces. They had throughout their whole mass the appearance 

 of paraffine, a distinct cleavage, probably basal as will be shown 

 later on, and a vitreous luster. The specific gravity was found 

 to be 1'692 (Keeley) and 1*54 (Stackhouse), the former being 

 probably too high owing to a slight decomposition caused by 

 boiling the specimen in water. An analysis by Mr. Keeley 

 gave the following formula : 3MgC0 3 . Mg(OH) 2 . 2lH 3 0, corres- 

 ponding to 



Found. Calculated. 



CO, 18-90 19-19 



Mo'O '23-18 23-25 



HO 57-79 57-56 



99-87 100-00 



In the month of June, 1888, Mr. Keeley paid a second visit 

 to the locality in order to secure the balance of the lansfordite, 



*" On the feasibility of establishing a light-wave as the ultimate standard of 

 length:" liicbelson and Morley, this Journal, xxxviii. Sept.. 1889. 

 fDr. Genth: Zeitschr. Kryst,. xiw 255. 



